Just a note as everyone else has covered the technical issues of this. In the Jegs Catalog that I have, the Double Pumpers are listed in a separate group and it says Double Pumper on the top. Then they list the carb. numbers and sizes. The actual heading as I look at the book is " Classic Double Pumper" and it then gives all the sizes from 600 cfm through 850 cfm, in either the normal copper finish or the
zinc finish. Some not in that group are double pumpers and will list the size of the primary and secondary pumps. Others will say mechanical secondaries, which aren't double pumpers.
Ron